Photographic emulsions contain grains comprised of one or a combination of silver chloride, silver bromide and silver iodide. Although these photographically useful silver halides are the sole silver salts employed for grain formation in the overwhelming majority of photographic applications, silver salts, such as silver thiocyanate, silver phosphate, silver pyrophosphate, silver cyanide, silver citrate and silver carbonate, can be incorporated in the grains in addition to the silver halide(s), as illustrated by Berriman U.S. Pat. No. 3,367,778; Maskasky U.S. Pat. Nos. 4,435,501, 4,463,087, 4,471,050 and 5,061,617 and Research Disclosure, Vol. 181, May 1979, Item 18153; Ikeda et al U.S. Pat. No. 4,921,784 and Ihama et al EPO 0 312 959. Research Disclosure is published by Kenneth Mason Publications, Ltd., Dudley House, 12 North St., Emsworth, Hampshire P010 7DQ, England.
In recent years photographic interest has focused tabular grain silver halide emulsions. An emulsion is generally and for the purposes of this invention considered to be a "tabular grain emulsion" when tabular grains account for at least 50 percent of total grain projected area. A grain is generally and for the purposes of this invention considered to be tabular when the ratio of its equivalent circular diameter (ECD) to its thickness (t) is at least 2. The equivalent circular diameter of a grain is the diameter of a circle having an area equal to the projected area of the grain. Grains in which the ratio of adjacent major face edge lengths are 10 or more are classified as rods rather than tabular grains.
Maskasky U.S. Pat. Nos. 5,264,337 and 5,275,930, House et al U.S. Pat. No. 5,320,938, and Brust et al published European Patent Application 0 534 395 disclose radiation sensitive high chloride {100} tabular grain emulsions. As employed herein the term "high chloride {100} tabular grain emulsion" indicates a high chloride tabular grain emulsion in which the tabular grains accounting for at least 50 percent of total grain projected area have major faces lying in {100} crystallographic planes. The high chloride {100} tabular grain emulsions of Maskasky, House and Brust et al represent an advance in the art in that (1) by reason of their tabular shape, they achieve the known advantages of tabular grain emulsions over nontabular grain emulsions, (2) by reason of their high chloride content they achieve the known advantages of high chloride emulsions over those of other halide compositions (e.g., rapid development and increased ecological compatibility--that is, rapid processing with more dilute developer solutions and rapid fixing with ecologically preferred sulfite ion fixers), and (3) by reason of their {100} crystal faces the tabular grains exhibit higher levels of grain shape stability, allowing the use of morphological stabilizers adsorbed to grain surfaces during emulsion preparation to be entirely eliminated. A further and surprising advantage of high chloride {100} tabular grain emulsions has been their sensitivity levels, which can be higher than previously thought possible for high chloride emulsions.
Since silver chloride exhibits lower native absorption in the blue region of the spectrum than the remaining photographic silver halides, silver bromide and silver iodide, the high chloride {100} enjoy an advantage over silver bromide and iodobromide tabular grain emulsions when employed in color photography to record minus blue (green and/or red) exposures, but a disadvantage when employed to record blue exposures. Although blue absorbing spectral sensitizing dyes can be employed, the lack of native blue sensitivity puts these emulsions at a disadvantage when used to record blue light exposures in either black-and-white or color photography.
Brust and Mis U.S. Pat. No. 5,314,798, commonly assigned incorporated into high chloride {100} tabular grains increased iodide bands. These bands improved the speed-granularity characteristics of the emulsions. The higher iodide also increased native blue sensitivity. Unfortunately, silver iodide is the least soluble of the photographic silver halides, slows development rates and is ecologically more burdensome than the other photographic halides. A further disadvantage of iodide when incorporated in high chloride {100} tabular grain emulsions is that iodide increases native blue absorption primarily in the short blue (400 to 450 nm wavelength) region of the blue spectrum rather than the long blue (450 to 500 nm) region of the spectrum that is of greater photographic interest.